Ok, well it depends what you mean by "compatible with". You
can purely use a surround sound system as a hifi, and vice versa.
1. If you want to use it as a surround sound system primarilly, go for a suround sound system and then attach a cd player or an mp3 player to it when you want to use it as a hifi.
Pros: Works great as a surround sound system, with occasional use as a normal stereo
Cons: Sometimes not as good sound quality for music playback as a normal hifi
2. Get a small cheap hifi with an aux input, and attach 2 extra speakers.
Pros: Slightly better sound quality than a surround sound system
Cons: Will probably cost more than a cheap surround sound system if you want good quality, won't work as a proper surround sound system as it as no decoder and so will blast the same sound out of every speaker
3. Get both a cheap hifi and a cheap surround sound system
Pros: You've covered every angle, sort of
Cons: It's better to have one good speaker system that 2 bad ones, Systems probably won't be compatible because cheap cost normally means lack of inputs.
I'd personally go for option number 1, and get the best surround sound system you can for the price. If you want to use it for proper surround sound,
Make Sure You Get One With A Decoder, and not just a speaker system that requires one. If you're plugging it into a simple thing like terestrial TV, look out for a virtual surround sound setting, which means the system will do it's best to separate out the signal into the correct parts for the front and rear speakers. That's where i messed up.
If you go for the 2nd option, you'll need to get the best hifi you can for about £45, and then find yourself an additional set of bookshelf speakers to plug into it for about £15-20. I wouldn't reccomend doing that though if you want proper surround sound.
If you went for option 3, you'd probably end up with a dreadful hifi, and a dreadful surround sound system. The cheapest surrond sound system you're gonna find is £40, go below that and you're just buying computer speakers. So, that leaves you with £20 to spend on a hifi, which gets you a nasty tinny sounding little alba or mikomi, like
this.
Going on option one, there's 2 sub options (

)
1. Surround sound with a DVD player
Some have other inputs for stuff like TVs and PS2s, some don't. They need to be looked into heavily before buying one, as some are purely for playing dvd in surround sound on the cheap, and nothing else. But, they're cheaper

My pick would be
this. There are loads around at this pricerange, so i picked this from argos.
2. Surround sound speaker systems (with decoder, mind)
Practically guaranteed to have inputs for just about everything, but you can't find them so easily on the high street, and they cost a bit more. I'd choose
this, or
this. They cost a bit more, but they're far more likely to work, and sound great. If you want a cheaper, try
this.
Those are only suggestions, so if you find anything tempting, post up the details and i'll give it the once over for you
Hope this helps
Jon